1. Field of the Invention
The present invention relates to an optical recording medium wherein light is used to read out recorded data, particularly to an optical recording medium whose surface is not easily injured.
2. Related Art
Conventionally, a resin such as polycarbonate or PMMA is naked to the surface of an optical disc. Thus, it is f eared that the surface is injured when the optical disc is handled. When dirt adhering to the surface of the optical disc is wiped out, the surface of the optical disc is rubbed by the adhering dirt so that the surface may also be injured. In the case that signals are read out through a pickup or signals are recorded on the optical disc, such an injury causes deterioration of the readout signals or recorded signals. Therefore, about recording-type optical discs which are affected by injuries upon both of recording and reproducing, such as DVD-RAM, MD and MO, it is essential that the optical discs are put into cartridges when used.
On the other hand, about ROM type optical discs for reproducing use only, no cartridge has been required up to now since fine injuries in the surface thereof do not produce fatal effects on reproducing.
In recent years, digital high-vision televising has been prepared to be carried out through BS digital broadcasting. For this, a disc in which digital high-vision signals can be recorded for two hours is demanded. The transmitting rate of digital high-vision is from 19 to 23 Mbps. In order to record signals thereof for two hours or more, a recording capacitance of 17.1 Gbytes or more is required for 19 Mbpsxc3x9760 secondsxc3x97120 minutes or more conditions in the case that the transmitting rate is 19 Mbps. A recording capacitance of 20.7 Gbytes or more is required for 23 Mbpsxc3x9760 seconds 120 minutes or more conditions in the case that the transmitting rate is 23 Mbps. This is 3.64-4.4 times larger than the recording capacitance of digital versatile discs (DVDs), that is, 4.7 Gbytes. In order to realize reproducing which is not less than 3.64 times denser than reproducing of DVDs, it is necessary to raise the resolution for reproducing, that is, make the spot diameter of the reproducing beam used smaller. The spot diameter xcfx86 of the reproducing beam is represented by the following: xcfx86=k xcex/NA wherein the wavelength of the reproducing light source used is xcex, the numerical aperture of the objective lens used is NA, and k is a constant. For example, by changing the wavelength of the reproducing light source from 650 to 405 nm, the reproducing resolution is improved by 1.6 times. By changing the numerical aperture of the objective lens from 0.6 to 0.85, the reproducing resolution is improved by 1.42 times. Consequently, it becomes possible to reproduce a high-capacity disc whose recording capacitance becomes larger by about 5.16 times [(1.6)2xc3x97(1.42)2].
In order to satisfy a recording capacitance that is not less than 3.64 times larger than that of DVDs, it is necessary that the wavelength of the reproducing light source is 420 nm or less and the numerical number of the objective lens is 0.74 or more.
In the above-mentioned disc, the substrate thereof must be made far thinner than that of DVDs since the disc is highly affected of aberration at the same tilt angle because of the rise in NA. In this case, however, injuries in the surface of the disc or dirt thereon is nearer to the signal surface thereof than in the prior art; therefore, the injuries or the dirt produces a larger effect on recording and reproducing property. Therefore, in the case that a conventional optical disc structure is used, it can be expected that a situation that the optical disc must be put into a cartridge when used is caused.
Concerning a ROM type optical disc, however, it is necessary to consider that the disc is attached to a publication (for example, book) and is used as a part of the publication. Thus, when such a use situation is supposed, an increase in the volume of the optical disc on the basis of the storage thereof into a cartridge or a rise in costs for producing the disc is a fatal drawback. It is therefore demanded that an optical disc wherein data can be very densely recorded can be used without being put into any cartridge. Concerning a recording-type optical disc, for example, an advantage that the use situation thereof can be expanded can be produced by removing the necessity that the disc is put into a cartridge.
An object of the present invention is to provide an optical recording medium whose surface is not easily injured, particularly an optical recording medium which is not required to be put into a cartridge.
The optical recording medium of the present invention comprises a recording layer, a covering layer comprising a polymer material, and a protective layer which are successively deposited, recording or reproducing wavelength for the medium being 420 nm or less, the numerical aperture of an optical system for the medium being 0.74 or more, and light being radiated to the side of the protective layer to read out data in the recording layer, wherein the protective layer has a hardness of 1000 kg/mm2 or more and a thickness of 4.66 xcexcm or less.
This optical recording medium has the protective layer having a hardness of 1000 kg/mm2 or more. Therefore, the recording medium is suitable for high-density recording, and its surface is not easily injured. The optical recording medium has a recording density of not less than 3.64 times larger than that of DVDs, and is practical.
The thickness of the covering layer may have a thickness of 131 xcexcm or less. In this case, a recording density which is 3.64 times larger than that of DVDs can be attained.
The protective layer may have a light transmissivity of 90% or more to the recording or reproducing wavelength. In this case, restrictions about optical properties which the protective layer supplies to other layers can be made sufficiently small.
The protective layer may comprise an inorganic substance.
The protective layer may comprise diamond-like carbon. In this case, sufficient hardness and flexibility can be given to the protective layer.
The protective layer may comprise a metal oxide. In this case, the protective layer may comprise SiO2 or TiO2.
The protective layer may comprise a metal nitride.
The optical recording medium may be a read-only recording medium.
The optical recording medium producing method of the present invention is a method for producing an optical medium comprising a recording layer, a covering layer comprising a polymer material, and a protective layer comprising diamond-like carbon which are successively deposited, data in the recording layer being read out from the side of the protective layer, comprising the process of forming the covering layer on a surface of the recording layer, and the process of forming the protective layer on the surface of the formed covering layer by vapor deposition.
According to the present method for producing the optical recording medium, the protective layer comprising diamond-like carbon is formed; therefore, the produced recording medium is suitable for high-density recording and its surface is not easily injured. Moreover, sufficient flexibility can be given to the protective layer.
Sputtering may be used in the process of forming the protective layer.
Carbon may be used as a target and a gas comprising argon may be used as a sputtering gas in the process of forming the protective layer.
A gas obtained by mixing argon with at least one selected from hydrogen gas, methane gas and nitrogen gas may be used as the sputtering gas.